Two-dimensional angle of arrival detection device

ABSTRACT

A device for detecting the direction of a light source. The device has a pin-hole lens that allows a collimated light beam to excite a light sensing surface behind the lens. The output from the light sensing surface is passed to a processor that determines the position of the surface that has been excited and the direction of the light source. When the position of one or more light sources is known, the device may further determine its own position. The devices may be used in a location system to provide known reference points to a network of other devices. The light sources may be modulated, in which the device can select or identify a particular light source based upon its modulation pattern.

TECHNICAL FIELD

[0001] This invention relates to techniques and apparatus for detectingthe two-dimensional angle of arrival or direction of an optical source.This invention is especially suitable for use in a location estimationsystem utilizing optical transmitters.

BACKGROUND OF THE INVENTION

[0002] Outdoor location or positioning systems, such as RADAR, GPS andLORAN, have been used for many years to locate people or objects. Thesetechniques generally make use of radio frequency (RF) signals from atransmitter. Various properties of the RF signal can be measured todetermine location; these include Receive Signal Strength (RSS), TimeDifference of Arrival (TDoA) and Angle of Arrival (AoA). However, thetechniques have limited accuracy and often require expensive supportinginfrastructure. Moreover, they cannot be used effectively insidebuildings because the RF transmission signals undergo fading, dispersionand reflection. Reflections cause the signal to arrive at a detectorfrom multiple paths and create multipath interference. However,techniques and apparatus for accurate positioning of people and objectswithin buildings are required for applications such as personal securityand asset management systems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The features of the invention believed to be novel are set forthwith particularity in the appended claims. The invention itself however,both as to organization and method of operation, together with objectsand advantages thereof, may be best understood by reference to thefollowing detailed description of the invention, which describes certainexemplary embodiments of the invention, taken in conjunction with theaccompanying drawings in which:

[0004]FIG. 1A is a diagrammatic representation of a network of deviceswith relative position knowledge.

[0005]FIG. 1B is a diagrammatic representation of a network of devicescombined with reference detection devices of the present invention.

[0006]FIG. 2 is a diagrammatic representation of a two-dimensional angleof arrival detection device of the present invention.

[0007]FIG. 3 is a diagrammatic representation of mapping betweenpositions on a light sensing surface and a two-dimensional angle ofarrival according to an embodiment of the present invention.

[0008]FIG. 4 is a diagrammatic representation of location systemincorporating a detection device of the present invention.

[0009]FIG. 5 is a flow chart of an embodiment of the location techniqueof the present invention.

[0010]FIG. 6 is a diagram showing a geometry for location calculation inaccordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] While this invention is susceptible of embodiment in manydifferent forms, there is shown in the drawings and will herein bedescribed in detail specific embodiments, with the understanding thatthe present disclosure is to be considered as an example of theprinciples of the invention and not intended to limit the invention tothe specific embodiments shown and described. In the description below,like reference numerals are used to describe the same, similar orcorresponding parts in the several views of the drawings.

[0012] The present invention relates to device that can determine atwo-dimensional angle of arrival of light falling upon the device. Bydetermining the angles for light incident from a number of sources, thedevice can determine its own position relative to the sources. Inaddition, if the locations of the sources are known, the device candetermine its own absolute position.

[0013] An example use of the device is in concert with an opticalcommunications infrastructure in a building. The device can determineits own position relative to the infrastructure and provide a referencepoint for other in-building location devices. In many cases an opticalinfrastructure will consist of previously installed, in-building lights.Such lights might include incandescent bulbs, fluorescent lamps andhalogens, LED's or laser diodes. Other in-building location devices,which will be referred to as “blind devices”, typically consist of asensor, an RF transceiver and a processor that can perform positionestimates based upon signal strength or some other ranging technology.The device of the present invention may be used to enhance such aninfrastructure by providing an absolute position reference to thelocation map created by the blind devices.

[0014] Blind devices include sensors for detecting temperature changes,chemical irregularities, moisture or even dangerous situations. In orderto make use of the information gathered by the sensor, the device mustcommunicate with its neighbors and determine the distance from all orsome of its nearest neighbors. By doing this, the estimated location ofthe event can be determined and recorded and the appropriate actionstaken. Once every blind device has recorded the position of thesurrounding devices, the position of each device relative to the networkis known. This can be calculated by each device or by a centralprocessor that communicates with the devices. That is to say, a map canbe created that shows the location of every device relative toneighboring devices, but the map does not have a reference point.

[0015]FIG. 1A, shows a map of devices, labeled 1-6, in a officebuilding. The device can typically detect the distances d_(ij) to otherdevices, and thereby determine their position relative to the otherdevices. However, the absolute positions of the devices within an officebuilding are unknown unless, as shown in FIG. 1B, a number of referencedevices are added to the network. FIG. 1B shows a plan view of thelocation of the same network of devices with respect to the fixedstructure of an office building. The reference devices, shown as squareslabeled A, B and C in the figure, allow all of the other devices todetermine their absolute positions (x_(i), y_(i), z_(i)), as shown inFIG. 1B. This is possible because the reference devices, A, B and C, areable to determine their absolute positions (x_(A), y_(A), z_(A)) etc.,with respect to the fixed infrastructure of the office building.

[0016] An important feature of the present invention is the ability ofthe device to be placed in an environment and determine its absoluteposition without the need for manual configuration. This ability reducesthe cost of setting up a location infrastructure. Previously, referencedevices would have to be configured manually or have their positionsrecorded.

[0017] A diagrammatic representation of the detection device of thepresent invention is shown in FIG. 2. Referring to FIG. 2, the detectiondevice 100 includes a pin-hole lens 102 with effective aperture 104, alight sensing surface 106, such as a CCD array, and a processor 108. Inthe preferred embodiment, the output from the light sensor 106 issampled by an analog-to-digital converter (ADC) (not shown) and theresulting digital signal is passed to the processor 108. The plane ofthe pin-hole lens and light sensor is designated as the x-z plane. They-axis is perpendicular to the x-z plane, as shown in the figure. Thedistance between the pin-hole lens 102 and the light sensing surface 106is denoted by d. Light incident on the pin-hole lens 102 at an angle θto the plane of the lens and an angle φ to an x-axis in the plane of thelens produces a collimated beam that excites the light sensing surface106 at a position 110 a distance r from the y-axis. The position 110 isdenoted by cylindrical coordinates (r, φ) or by Cartesian coordinates(x₁, z₁), these are related by r={square root}{square root over (x₁ ²+z₁²)} and$\varphi = {{\arctan \left( \frac{- x_{1}}{z_{1}} \right)} \cdot}$

[0018] In operation, the coordinates (x₁, z₁) are measured from whichthe distance r and the angle φ can be calculated. In addition, since thedistance d between the light sensing surface and the pin-hole lens isknown, the angle θ can be calculated as$\theta = {{\arctan \left( \frac{d}{r} \right)} = {{\arctan \left( \frac{d}{\sqrt{x_{1}^{2} + z_{1}^{2}}} \right)}.}}$

[0019] The functions${\varphi \left( {x_{1},z_{1}} \right)} = {{{\arctan \left( \frac{- x_{1}}{z_{1}} \right)}{and}\quad {\theta \left( {x_{1},z_{1}} \right)}} = {\arctan \left( \frac{d}{\sqrt{x_{1}^{2} + z_{1}^{2}}} \right)}}$

[0020] can be tabulated in a look-up table in the processor orcalculated as required. The detection device is thus able to determinethe direction (θ, φ) of the light incident upon the device.

[0021] The direction can also be determined as a vector direction. Forexample if we denote the location of the pin-hole as x=(x,y,z) and thelocation of the reference light source as xi=(x_(i), y_(i), z_(i)), thedirection is defined by the unit vector n_(i)$n_{i} = {\frac{x_{i} - x}{\left| {x_{i} - x} \right|}.}$

[0022] Light incident from a direction close to the y-direction willfall close to center of the light sensing surface. Light incident atincreasing angles to the y-direction will fall further from the centerof the sensing surface. In the preferred embodiment, the distance d issmall relative to the size of the light sensing surface, so that a widerange of angles θ can be measured.

[0023] In a further embodiment, the light sensing surface 106 is concaveor curved in a bowl shape, so that a greater range of angles θ can bemeasured. The light sensing surface could also be convex.

[0024] In the preferred embodiment, the light sensing surface is dividedinto a matrix of individual photo-detector elements. Each element ismapped to a particular pair of angles (θ, φ). Thus, if a particularelement is excited by a column of light, it is known that the light camefrom a direction with angles (θ, φ).

[0025]FIG. 3 shows an example of how the angles θ and φ map onto thesurface of the light sensing surface. The concentric circles depictlines of constant θ, while the radial lines depict lines of constant φ.In this example, the surface is flat and square with sides of length 4d;i.e. four times the separation between the light sensing surface and thepin-hole lens. In this example, if light falls on the point labeled ‘A’in FIG. 3, then the light is determined to have been incident from thedirection θ=74°, φ=135°.

[0026] In a further embodiment the lens is a hologram that receiveslight from a light source and projects it onto the light sensingsurface. The position of projected image may be used to determine thedirection of the incident light.

[0027]FIG. 4 is a diagram showing an exemplary system incorporating adetection device 100 of the invention. In this example, a modulatedlight source 402 is controlled by control circuit 404. The controlcircuit controls aspects of the light source, such as the modulation,intensity and operation period. Some of the modulated light produced bythe modulated light source 402 is received by the lens 102 of thedetection device 100. The light then falls on light sensing surface 106that in turn produces a signal that is sampled by analog-to digitalconverter (ADC) 406. The resulting digital signal is passed to processor108. The processor detects the coordinates on the light sensing surfacethat has the greatest illumination and maps those coordinates to anglesθ and φ, using a look-up table for example. The light source isidentified by the characteristics of the signal. For example, thespectrum, modulation pattern, or time of arrival could be used todetermine which of a number of light sources has produced the light.Once the light source has been identified, it's known location can beretrieved from memory 408. Additionally, the intensity of the light canbe determined. If this is compared with the known intensity of thesource, the distance from the light source to the detector may becalculated by the processor. The process may be repeated for a number ofdifferent light sources. Once the angle to a number of light sources hasbeen determined, the processor can determine its absolute location.

[0028] Information in addition to the directions to the light sourcesmay be used to determine the location of the device. For example, theheight or orientation of the light sensing surface may be known inadvance. In some applications, the height of the detection device may beknown. In this case two or more light sources may be used to determinethe position of the device.

[0029] In addition to a pin-hole lens, light sensing surface andprocessor, the device may have a display for displaying the location ofthe device to a user, a memory for storing location information or oneor more transmitters for transmitting the location of the device toother devices, such as blind devices. The transmitters may use optical,radio, ultrasonic, infra-red or other transmission media.

[0030] The device may also include one or more receivers to receivecommunication signals from other devices.

[0031] The device may include one or more transmitters to transmitcommunication signals from other devices or to a central processor. Inthe latter case, the position of the projected image may be transmittedto the central processor, and the calculation of the direction of thelight source or the position of the detection device may be performed bythe central processor.

[0032] The device may include a mirror or prism or other optical elementin the optical path between the lens and the light sensing surface: theoptical element serving to direct the light from the lens to the lightsensing surface.

[0033] The device may be incorporated in other devices such astelephones, Personal Digital Assistants (PDAs), computers or any devicethat needs to be tracked or located.

[0034] The optical properties of the pin-hole lens and the light sensingsurface may be selected to be more responsive to light with apredetermined spectrum, thereby making the device less sensitive toother light sources that might otherwise cause interference and reducethe accuracy of the device. In particular, the light sensing surface maycontain plurality of elements, responsive to different optical spectra.The processor may operate to select those elements responsive to thespectrum of a particular light source, thereby excluding the response ofother light sources. In this way the device may select between lightsources that are simultaneously active.

[0035] A further method for selecting between or identifying lightsources is the use of modulation. If each light source is modulated witha different modulation signal, the processor can correlate or demodulatethe signal from the light sensing surface with a selected demodulationsignal. Again, this allows the device to select between different lightsources. Selection is important when the directions to multiple lightsources must be measured to determine the location of the device.

[0036] A still further method for selecting between or identifying lightsources is the use of time division multiplexing. According to thismethod, each light source is active for a specified period or at aspecified time. Synchronization may be used to time-align the system.

[0037] The processor may perform matched filtering, enabling the deviceto respond to light with a predetermined modulation pattern; therebymaking the device less sensitive to other light sources that mightotherwise cause interference and reduce the accuracy of the device.

[0038] A flow chart depicting the method of the invention is shown inFIG. 5. Following start block 502, light from one or more light sourcesexcites the light sensing surface at block 504. The resulting signalsare sampled by an ADC and passed to the processor. The processor detectslight from a particular source, by the appropriate demodulation forexample, at block 506 and then retrieves the location of the lightsource from memory at block 508. At decision block 510 a check is madeto determine if more light sources are to be detected. This will be thecase when a single source direction is not sufficient to determine thelocation of the device. However, if the distance to the source and theorientation of the detector are known, a single source may besufficient. If more light sources are to be detected, as depicted by thepositive branch from decision block 510, flow returns to block 506. Ifno more light sources are to be detected, as depicted by the negativebranch from decision block 510, flow continues to block 512, and theposition of the device is calculated from the measured directions toeach light source and any additional information (such as theorientation of the device, the height of the device or the distance fromthe device to the light source). Alternatively, the position of theprojected image can be transmitted to a remote processor where thecalculations are performed. At block 514, the location of the device maybe displayed, on a screen for example, stored in memory or transmittedto other devices. The process terminates at block 516. The process maybe repeated at predetermined intervals or whenever the device has beenrelocated.

[0039] When the orientation of the detection device is unknown, as forexample when the device is integrated with a cellular telephone, thepositions of the images of three light sources on the light sensingsurface may be measured and used to determine the location of thedevice. One method for making this determination will now be described.FIG. 6 shows two light source positions x_(i) and x_(j). The lightpasses through lens 102 and is projected onto light sensing surface 106to form images at positions x′_(i) and x′_(j). We denote the location ofthe pin-hole as x=(x,y,z) and the locations of the reference lightsources as x_(i)=(x_(i), y_(i), z_(i)). Using this notation, we canwrite the unknown distances from the light source i to the pin-hole as

r _(i) =|x _(i) −x|=[(x _(i) −x)²+(y _(i) −y)²+(z _(i) −z)²]^(1/2),

[0040] and the known distance between light source i and light source jas r_(ij)=|x_(i)−x_(j)|. These distances are related by

r _(ij) ² =r _(i) ² +r _(j) ²−2r _(i) r _(j) cos(ψ_(ij)),

[0041] where ψ_(ij) is the angle between the directions to source i andsource j. The angles ψ_(ij) can be calculated from the image positionsx′_(i) and x′_(j) and the known distance from the leans to the lightsensing surface. The orientation of the device is not required. Writingc_(ij)=cos(ψ_(ij)), we can use three light sources to get the equations

r ₁₂ ² =r ₁ ² +r ₂ ² −r ₁ r ₂ c ₁₂

r ₁₃ ² =r ₁ ² +r ₃ ² −r ₁ r ₃ c ₁₃

r ₂₃ ² =r ₂ ² +r ₃ ² −r ₂ r ₃ c ₂₃

[0042] These equations can be solved to give a quartic equation for r₁.Once r₁ is known, r₃ and r₃ can be found. Alternatively, the equationscan be solved directly for x=(x,y,z). We introduce the errors defined bythe equations,

e ₁₂({circumflex over (x)},ŷ,{circumflex over (z)})=r ₁ ² +r ₂ ² −r ₁ r₂ c ₁₂ −r ₁₂ ²

e ₁₃({circumflex over (x)},ŷ,{circumflex over (z)})=r ₁ ² +r ₃ ² −r ₁ r₃ c ₁₃ −r ₁₃ ²

e ₂₃({circumflex over (x)},ŷ,{circumflex over (z)})=r ₂ ² +r ₃ ² −r ₂ r₃ c ₂₃ −r ₂₃ ²

[0043] where ({circumflex over (x)},ŷ,{circumflex over (z)}) is anestimate of (x,y,z). The equations can be solved by minimizing the costfunction

J({circumflex over (x)},ŷ,{circumflex over (z)})=e ₁₂ ²({circumflex over(x)},ŷ,{circumflex over (z)})+e ₂₃ ²({circumflex over (x)},ŷ,{circumflexover (z)})+e ₂₃ ²({circumflex over (x)},ŷ,{circumflex over (z)})

[0044] with respect to the position estimate ({circumflex over(x)},ŷ,{circumflex over (z)}) using a standard search algorithm. Thecalculation may be performed on the processor of the device, or thepositions of the images on the light sensing surfaces may be transmittedto a central processor where the calculations are performed.

[0045] Those of ordinary skill in the art will recognize that thepresent invention has been described in terms of exemplary embodimentsbased upon a detection device including a pin-hole lens, a light sensingsurface and a processor. However, the invention should not be solimited, since the present invention could be implemented usingequivalent hardware components those described above and claimed below.

[0046] While the invention has been described in conjunction withspecific embodiments, it is evident that many alternatives,modifications, permutations and variations will become apparent to thoseof ordinary skill in the art in light of the foregoing description.Accordingly, it is intended that the present invention embrace all suchalternatives, modifications and variations as fall within the scope ofthe appended claims.

What is claimed is:
 1. A device for detecting the direction of a lightsource, the device comprising: a lens for receiving light from the lightsource and projecting a light pattern; a light sensing surface forreceiving the light pattern from the lens and producing an output signalin response thereto; and a processor responsive to the output signal;wherein the processor is operable to determine the direction of a lightsource from the output signal.
 2. A device in accordance with claim 1,wherein the lens is a pin-hole lens.
 3. A device in accordance withclaim 1, wherein the lens is a holographic lens.
 4. A device inaccordance with claim 1, wherein the light sensing surface comprises aplurality of light sensing elements.
 5. A device in accordance withclaim 1, wherein the plurality of light sensing elements includeelements sensitive to light in a plurality of spectra.
 6. A device inaccordance with claim 1, wherein the light sensing surface comprises aCCD array.
 7. A device in accordance with claim 1, wherein the lightsensing surface comprises a photo-diode array.
 8. A device in accordancewith claim 1, further comprising an analog-to-digital converter coupledto the light sensing array and the processor, and operable to convertthe output signal into a digital output signal and provide the digitaloutput signal to the processor.
 9. A device in accordance with claim 1,further comprising a display coupled to the processor.
 10. A device inaccordance with claim 1, further comprising a memory coupled to theprocessor.
 11. A device in accordance with claim 1, further comprising acommunication transmitter coupled to the processor.
 12. A device inaccordance with claim 1, further comprising a communication receivercoupled to the processor.
 13. A device in accordance with claim 1,wherein the processor includes a demodulator.
 14. A device in accordancewith claim 1, wherein the light sensing surface is flat.
 15. A device inaccordance with claim 1, wherein the light sensing surface is concave.16. A method for determining the direction of a light source, the methodcomprising: receiving light from the light source; projecting the lightonto a light sensing surface; determining the position of the projectedlight on a light sensing surface; and mapping the position to adirection.
 17. A method as in claim 16, wherein the mapping uses alook-up table indexed by the position of the projected light on thelight sensing surface.
 18. A method for determining the position of adevice the method comprising: receiving light from a light source;projecting the light onto a light sensing surface to form a projectedimage; receiving a signal from the light sensing surface, the signalbeing indicative of the position of the projected image on the lightsensing surface; retrieving stored information from a memory; anddetermining the position of the device from the signal and the storedinformation;
 19. A method in accordance with claim 18, wherein thestored information includes the location of the light source.
 20. Amethod in accordance with claim 18, wherein the stored informationincludes the vertical position of the device.
 21. A method in accordancewith claim 18, wherein the stored information includes the orientationof the device.
 22. A method in accordance with claim 18, wherein thestored information includes previously measured positions of a projectedimage from other light sources and the locations of those light sources.23. A method in accordance with claim 18, wherein the stored informationincludes previously measured directions to other light sources and thelocations of those light sources.
 24. A method in accordance with claim18, further comprising: measuring the intensity of the collimated beam;determining the distance from the device to the light source; andstoring the distance in the memory, wherein the stored informationincludes the distance.
 25. A method in accordance with claim 18, furthercomprising transmitting the position of the device to other devices in anetwork of devices.
 26. A method in accordance with claim 18, furthercomprising displaying the position of the device on a display.
 27. Amethod in accordance with claim 18, further comprising storing theposition of the device in the memory.
 28. A method in accordance withclaim 18, wherein the determining the position of the device from thesignal comprises: selecting a demodulation signal corresponding to alight source; demodulating the sensor signal with the demodulationsignal to obtain a demodulated signal; determining the position of thecollimated beam on the light sensing surface in accordance with thedemodulated signal; and retrieving the location of the light source fromthe memory.
 29. A method in accordance with claim 18, wherein thedetermining the position of the device from the signal comprises:selecting a light source; selecting components of the sensor signal inaccordance with the spectrum of the light source to obtain a filteredsensor signal; determining the position of the collimated beam on thelight sensing surface in accordance with the filtered sensor signal; andretrieving the location of the light source from the memory.
 30. Amethod in accordance with claim 18, further comprising transmitting theposition of the projected image to other devices in a network ofdevices.
 31. A method in accordance with claim 18, further comprisingtransmitting the direction to a light source to other devices in anetwork of devices.
 32. An object location system comprising: a firstlight source located at a first known position; and a detection devicefor detecting light from the light source and determining the directionto the light source, the device comprising: a lens for receiving lightfrom the first light source and projecting a light pattern; a lightsensing surface for receiving the light pattern and producing an outputsignal in response thereto; and a processor responsive to the outputsignal; wherein the processor is operable to determine the position ofthe detection device from the output signal and the known position ofthe first light source.
 33. An object location system in accordance withclaim 32, wherein the lens of the detection device is a pin-hole lens.34. An object location system in accordance with claim 32, wherein thelight sensing surface of the detection device comprises a plurality oflight sensing elements.
 35. An object location system in accordance withclaim 32, wherein the first light source is modulated.
 36. An objectlocation system in accordance with claim 32, further comprising: one ormore additional light sources located at additional known positions;wherein the processor is operable to determine the position of thedetection device from the output signals due to each of the first andadditional light sources and the first and additional known positions ofthe light sources.
 37. An object location system in accordance withclaim 36, wherein the first light source and the additional lightsources are each modulated by a different modulation pattern.
 38. Anobject location system in accordance with claim 36, wherein the firstlight source and the additional light sources each emit light with adifferent spectral character.
 39. An object location system inaccordance with claim 36, wherein the first light source and theadditional light sources each emit light at different times.
 40. Anobject location system in accordance with claim 32, further comprising:a controller coupled to the first light source and operable to control acharacteristic of the first light source.
 41. An object location systemin accordance with claim 32, wherein the processor determines a distancefrom the detection device to the first light source according to theintensity of the light falling on the light sensing surface.
 42. Anobject location system in accordance with claim 32, wherein the lightsource is one of an incandescent bulb, a fluorescent lamp, an LED and alaser diode.
 43. An object location system in accordance with claim 32,wherein said detection device further comprises a first transmitter fortransmitting the location of the detection device, said system furthercomprising: a plurality of sensing devices, each sensing devicecomprising: a second transmitter for transmitting signals to othersensing devices; a receiver for receiving signals from other secondtransmitters and from the first transmitter; and a processor coupled tothe second transmitter and the receiver, wherein each of the pluralityof sensing devices is operable to determine its position relative toother sensing devices and relative to the detection device.
 44. Anobject location system in accordance with claim 43, wherein a sensingdevice of the plurality of sensing devices includes further comprises asensor for sensing a property selected from the group consisting oftemperature, moisture, chemical composition, pressure, motion and lightintensity.
 45. An object location system in accordance with claim 32,wherein the detection device is incorporated in an object that is to betracked wherein the detection device further comprises a transmitter fortransmitting the location of the detection device.
 46. An objectlocation system in accordance with claim 45, further comprising: areceiver for receiving the location of the detection device transmittedfrom the transmitter of the detection device.
 47. An object locationsystem comprising: a first light source located at a first knownposition; a detection device for detecting light from the light source,the device comprising: a lens for receiving light from the first lightsource and projecting a light pattern; a light sensing surface forreceiving the light pattern and producing an output signal in responsethereto; a device processor responsive to the output signal and operableto determine the position of the light pattern on the sensing surface;and a transmitter for transmitting the position of the light pattern onthe sensing surface; and a receiver for receiving the position of thelight pattern on the sensing surface; and a central processor coupled tothe receiver and operable to determine the location of the detectiondevice. from the receiving the position of the light pattern on thesensing surface and the known position of the first light source.